Level measurement by means of a radar is an elegant, precise and reliable method. This well-established technique uses, for example, horn antennas exciting the TE11-fundamental mode (also known as H11-mode) in the circular wave guide, propagated in bypass tubes. Horn antennas and the use of the fundamental TE11-mode allow high resolution and high accuracy, but there are limitations due to the influence of the wall material of the measuring pipes. Level detection of products with a low relative permittivity or under extreme conditions (e.g. pressure or temperature) in industrial tanks often requires bypass pipes or stand pipes. The bypass holes may cause false echoes, disturb the measurement and may decrease the accuracy.
Hence, there is a need for an antenna system which can be used in tubes, for example, bypass tubes, for measuring the filling height of a filing good in a receptacle and which has at least an accuracy as can be achieved by usage of a horn antenna or an even better accuracy.
A level measuring device comprising a planar antenna is, for example, shown in WO 02/31450 A1. This planar antenna comprises a plurality of straight metallic portions extending radially from a center and having arms connected with the straight portions and extending tangentially on the perimeter of a circle. All arms extend in the same direction. All these elements are arranged on the same surface of a substrate. It is outlined that such a structure would be advantageous with respect to the minimum clearance (also known as block distance) between the planar antenna and a free surface of a filling good of which the filling height is to be measured, because the disclosed planar antenna would reduce the block distance.